The cmaR gene of Corynebacterium ammoniagenes performs a novel regulatory role in the metabolism of sulfur-containing amino acids

Abstract

A novel regulatory gene, which performs an essential function in sulfur metabolism, has been identified in Corynebacterium ammoniagenes and was designated cmaR (cysteine and methionine regulator in C. ammoniagenes). The cmaR-disrupted strain (Delta cmaR) lost the ability to grow on minimal medium, and was identified as a methionine and cysteine double auxotroph. The mutant strain proved unable to convert cysteine to methionine (and vice versa), and lost the ability to assimilate and reduce sulfate to sulfide. In the Delta cmaR strain, the mRNAs of the methionine biosynthetic genes metYX, metB and metFE were significantly reduced, and the activities of the methionine biosynthetic enzymes cystathionine gamma-synthase, O-acetylhomoserine sulfhydrylase, and cystathionine beta-lyase were relatively low, thereby suggesting that the cmaR gene exerts a positive regulatory effect on methionine biosynthetic genes. In addition, with the exception of cysK, reduced transcription levels of the sulfur-assimilatory genes cysIXYZ and cysHDN were noted in the cmaR-disrupted strain, which suggests that sulfur assimilation is also under the positive control of the cmaR gene. Furthermore, the expression of the cmaR gene itself was strongly induced via the addition of cysteine or methionine alone, but not the introduction of both amino acids together to the growth medium. In addition, the expression of the cmaR gene was enhanced in an mcbR-disrupted strain, which suggests that cmaR is under the negative control of McbR, which has been identified as a global regulator of sulfur metabolism. DNA binding of the purified CmaR protein to the promoter region of its target genes could be demonstrated in vitro. No metabolite effector was required for the protein to bind DNA. These results demonstrated that the cmaR gene of C. ammoniagenes plays a role similar to but distinct from that of the functional homologue cysR of Corynebacterium glutamicum.